The inventive disclosure contained herein relates to various applications of temperature measurement and control via electric heaters (or in some cases cooling configurations) abound throughout all of industry, from consumer goods such as electric blankets to petrochemical processing, to spacecraft thermal control. These diverse applications are tied together by the need to measure a temperature and provide controlled electrical power for heating and/or cooling based on that temperature. A heater and/or cooler, temperature sensing device, and the thermally conductive structure connecting them can be referred to as a “thermal zone.” In a thermal zone, a single temperature measured or used to control a heater is representative of the temperature of the zone as a whole.
The use of mechanical thermostats, such as snap-action bimetallic thermostats, is a common way to control the temperature of a thermal zone using only two wires between the zone and a power source. However, these suffer the drawback of a fixed, low-accuracy temperature setpoint, and provide no means for the user to monitor the temperature. Another type of two-wire solution that has many examples in the patent literature involves sensing a remote temperature by measuring the variation in resistance of the heating element. For example, see U.S. Pat. No. 3,789,190 to Orosy et al.; U.S. Pat. No. 3,869,597 to Strange; U.S. Pat. No. 4,086,466 to Scharlack; U.S. Pat. No. 4,554,439 to Cross et al.; and U.S. Pat. No. 4,636,619 to Sugimori. This approach suffers the drawbacks of poor accuracy due to biases caused by wiring/harness resistance, high/excessive power consumption during temperature measurement, limitations on the useful range of allowable heater resistance, and limitations on the heater material that can be used to those which significantly change resistance over the temperature range of interest. Because of the aforementioned difficulties with measurement accuracy, this approach is typically limited to applications in which an absolute temperature measurement is not necessary, and the heater setpoint can be selected with an in-situ calibration.
If a remote sensing and control application requires accurate absolute remote temperature monitoring through wires with significant resistance, and an electric heater, then four or more wires per thermal-control zone are typically necessarily needed, at least one pair of wires for the electrical heater power and at least two more wires to communicate the temperature-sensor readings. See, e.g., U.S. Pat. No. 6,894,254 to Hauschultz and U.S. Pat. No. 8,541,716 to Gu et al. The drawback with this approach is that using more than two wires between a central controller and its various remote thermal-control zones adds cost, size, and mass which is especially critical for mass-sensitive applications in aviation and aerospace.
Furthermore, another common issue in the present art is that regardless of whether two or more wires are used for the powering and temperature control of a system between a central controller and a remote zone, present solutions generally rely on voltage signals to communicate a parameter such as temperature from remote sensors. The problem with this is that as such sensors are more remote, the length of the wiring required becomes greater, which makes such signals more vulnerable to outside inductive forces to create interfering noise. In addition, the electrical resistance of longer wiring runs can also directly degrade the accuracy of the measurement, because the resistance of the wiring cannot be distinguished from the resistance of the sensor. While one solution to this is often shielded wiring, such a solution increases the cost and weight of the system, which is undesirable in many applications, especially in aerospace vehicles.
Consequently, there exists a need for a simplified temperature-control system to provide accurate temperature measurement from an electrically remote location that is unaffected by variations in harness resistance and undesired induced voltage-signal “noise” from extra-system sources, that reduces the overall system mass, and that can provide heater power to that electrically remote location, all by using no more than using only two wires to transmit power and temperature-sensor signals.